Overt security elements including watermarks, metallic threads and optically variable devices such as holographic foils have been used for some time to authenticate documents, bank notes and other financial transaction instruments, such as credit and debit cards, for protection against copying and counterfeiting. Such security elements are overt in that their presence is visible to the naked eye. Covert security features, such as those which have luminescent properties are also used for authentication. In this case, the security feature is normally hidden under ambient light and is only revealed to the naked eye when illuminated by a special light source such as a UV lamp. Covert security features may also include features which can only be detected by a machine, such as those which emit outside the visible spectrum or are based on magnetic or electrical properties of a material.
Security features may be classified as “human unassisted”, in which the security feature is visible to the naked eye and can be authenticated by a human without machine assistance, “human assisted” which is defined as one in which the authentication process is performed by a person with the assistance of a tool or device, and “machine readable” in which the security feature is both detected and its authentication processed by a machine.
One of the most common human assisted features found on bank notes is the ultra-violet (UV) fluorescent feature. This feature is typically applied as an ink, which may be visible or invisible, by offset printing and usually forms an image made up of one to three colours (red, green, blue). The image is detected by exposure of the bank note to a UV light source (typically UVA at 365 nm). This feature has provided a reasonably good level of security against most primitive and hobbyist type counterfeiting in the past. However, the availability of UV fluorescent inks in red, green and blue colours, combined with the growing popularity of the Internet and the widespread access to ink jet printers, has severely eroded the security of the feature to the point where the feature can now be considered a liability. Accordingly, there is a need for an alternative security feature which is more robust against copying and counterfeiting.
U.S. Pat. No. 6,036,232 (Kaule et al.) discloses a data carrier in which a layer of fluorescent lacquer is provided on the carrier substrate and is overlaid with an optically variable element in the form of a holographic stripe. The fluorescent layer extends beyond the area of the optical element and is unobstructed to enable both the fluorescent layer and the optical element to be detected from the same side of the document.
U.S. Pat. No. 6,165,592 (Berger et al.) discloses a document with a security attribute having a diffractive optical element, which can be detected visually, and a luminescent characteristic, which can be detected by machine. The attribute comprises a composite having a transparent layer, certain regions of the lower surface thereof being embossed with a diffraction pattern and metallized, and an adhesive layer adjacent the lower surface of the transparent layer, which is doped with a luminescent substance in regions opposite non-metallized portions of the transparent layer to allow visual access to the luminescent substance through the transparent layer. In an alternative embodiment, the luminescent substance is provided in certain regions of the transparent layer.
U.S. Pat. No. 7,040,663 (Plaschka et al.) discloses a value document which includes a security element having an optically variable material which conveys different colour effects at different viewing angles. The security element also includes a machine-readable feature substance such as a luminescent substance, which luminesces outside the visible spectral region, with narrow emission bands that can be detected by machine. The machine readable substance does not impair the visually visible optically variable effect of the optically variable material. In one example, the security feature includes a dark-coloured print formed of black, IR-transparent ink containing a luminescent substance which emits in the IR spectral region. Disposed over the printed image is an optically variable material having no or little body colour. The material may be a liquid-crystal polymer or an interference layer material such as Iriodine™. In another example, the security element consists of a magnetic layer containing carbon black components, above which is disposed a translucent optically variable layer, which additionally contains luminescent substance. The luminescent substance can be transparent in the visible spectral region and emitting in the visible spectral region under UV illumination. The security element has a number of machine testable characteristics which include magnetic properties, luminescence emission under UV illumination and electric conductivity produced by the carbon black mixture.
US 2007/0273147 (Phillips et al.) discloses various vacuum roll coated security thin film interference products with overt and/or covert patterned layers. In one example, a multi-layered optical device is formed on a PET web, in which the device includes a patterned aluminum reflective layer on the front side of the web, overlaid by an optical interference structure including a spacer layer and an absorber layer, which, together with the reflective layer, form a Fabry-Perot resonant cavity. The aluminum layer is absent in discrete areas of the substrate to form windows. Another optical structure such as a reflective layer, an optically variable layer, a magnetic layer or a fluorescent layer is formed on the backside of the web. In another example, an aluminum pattern is applied to the front side of a PET web and an anti-Stokes fluorescent layer (which fluoresces at a shorter wavelength when illuminated by longer wavelength radiation) is disposed on the backside of the web. The fluorescent layer becomes visible through windows in the patterned aluminum layer when irradiated with IR light.
EP 1669213 (Schmid et al.) discloses a security element having a coating layer and an underlying substrate containing information, in which the coating layer contains similarly oriented pigment flakes to produce a “Venetian blind” effect, whereby at a perpendicular viewing angle the underlying substrate is hidden and at an oblique viewing angle corresponding to the orientation of the flakes, the underlying substrate is visible. The underlying substrate may contain printed indicia, an iridescent or optically variable ink, a luminescent material, a diffraction grating or thin-film interference layer.
EP 1719636 (Despland et al.) discloses a black-to-colour shifting security element comprising a layer containing colour shifting pigments overlaid by an absorber layer. The absorber layer is adapted to selectively absorb wavelengths of light emitted from the pigment layer at an orthogonal angle but not to absorb wavelengths emitted from the colour shifting pigment at grazing angles. This results in an effect whereby the colour shifting pigment, when viewed at an orthogonal angle, cannot be seen, and therefore the security element appears black, but can be seen when viewed at grazing angles. The absorber layer may be part of the pigment or disposed in a separate layer above the pigment layer.
US 2005/0240549 (Adamczyk et al.) discloses a data carrier which includes a security feature comprising an embossed structure which provides an optically variable element. The embossed structure has a line pattern or line screen structure, which, in cross section has side by side ridges with peaks and troughs joined by sloping sides or flanks. A first ink coating is provided on one side of each ridge and a second ink layer is disposed on both sides of the ridges above the first ink layer. On oblique viewing from the side which includes both coatings, a viewer will perceive an almost uniform colour print in the mixed colour from both coatings. When viewed from the opposite side, the viewer is faced by flanks of the ridges with the second coating only and will therefore receive a uniformly coloured surface only in the colour of the second coating so that the perceived colour changes depending on the side from which the embossed structure is viewed. The structure can be made machine readable by adding a feature substance such as a luminescent substance, a magnetic substance or an electrically conductive substance to at least one of the inks.
U.S. Pat. No. 6,210,777 (Vermeulen et al.) discloses a security document having a transparent or translucent support containing interference pigments. The interference pigments produce an effect whereby the document has a different colour when viewed with light transmitted by the document in comparison with light reflected by the document, and also has a different colour when viewed in transmission mode from the front or rear side. In one example, a pattern containing light interference pigments on one side of the transparent support is printed over a pattern containing light reflecting pigments of complementary colour to the interference pigments, a pattern of complementary pigments mixed with coloured fluorescent or phosphorescent pigments or dyes, and patterns containing solely fluorescent or phosphorescent dyes that are white by inspection with visible light but emit coloured light when exposed to ultraviolet light.
U.S. Pat. No. 6,565,770 (Mayer et al.) and U.S. Pat. No. 6,572,784 (Coombs et al.) disclose colour shifting pigments for inks and paints in which each pigment has a core comprising a multi-layer thin film structure which, under ambient light causes a viewing angle-dependent colour shift. Each pigment also includes a luminescent material as a secondary independent feature. In Mayer et al., the luminescent material encapsulates the optically variable core structure, whereas in Coombs et al., the luminescent material is incorporated within the core either as a separate layer or in one of the core layers.
U.S. Pat. No. 6,695,905 (Rozumek et al.) describes optically variable pigments for inks and coatings for security elements in which the pigment comprises a Fabry-Perot resonance cavity which provides a viewing angle-dependent colour shift under ambient light. In addition, a luminescence centre is incorporated into the resonance cavity. The cavity's resonance condition is dependent on the angle of incidence of the excitation light, and the luminescent material within the cavity is more strongly excited under the resonance condition than a luminescent material outside the cavity. This provides a means of machine-discrimination between the luminescent optically variable pigment and simple mixtures of luminescent materials and non-luminescent optically variable pigments.
U.S. Pat. No. 6,666,991 (Atarashi et al.) discloses a fluorescent or phosphorescent powder comprising spherical base particles covered with a multi-layered film coating to produce particular colours according to the refractive index and thickness of the various materials. A luminescent material is included as part of the powder's sphere either as a coating or an internal layer, for example, a layer next to the base particle. The luminescent material provides a means of authenticating the powder composition by detecting its presence when irradiated by a suitable light source.